Metallic miniaturized hollow shaped bodies and method for producing shaped bodies of this type

ABSTRACT

According to the invention, the shaped bodies are comprised of at least one heavy metal, preferably Fe, Ni, Co, Sn, Mo or W which can be reduced from a corresponding metallic compound at a temperature of less than 1 500° C. The shaped bodies have an outer diameter ranging from 0.05 to 0.5 mm, and a diameter-to-wall thickness ratio of 0.5 to 3%.  
     According to the method, starting materials are deposited as an enveloping layer on supporting elements of any shape, and the green compacts thus produced are subsequently heat-treated. On that occasion, the supporting elements are pyrolyzed, the enveloping layers are essentially thermally decomposed and the decomposition products are sintered. The outer dimensions of the supporting elements are selected such that they are larger than the shaped bodies to be produced. Metallic compounds, preferably metal oxides, metal hydroxides, metal carbonates or organometallic compounds are used as starting materials and can be reduced at a temperature of less than 1 200° C. The thermal treatment is carried out in a reductive atmosphere containing hydrogen and/or carbon such that the starting materials are essentially reduced to the sintered metal which is based on the respectively used metallic compound.

[0001] The invention relates to metallic miniaturized hollow shaped bodies according to claim 1. Furthermore, the invention relates to a method for producing metallic miniaturized hollow shaped bodies according to the preamble of claim 7.

[0002] The production of metallic oxidic or ceramic hollow balls is known for quite some time. However, the commercial utilization has been practically impossible for a long time. More recently, the fundamental object increasingly becomes more significant again. Advantageous possibilities of application of such shaped bodies will be seen for the most different purposes of design, e.g., in the lightweight construction, with crash absorbers, heat insulators and sound absorbers.

[0003] In practice, hollow balls are mainly fabricated since mostly the required supporting elements are easier to be obtained in the spherical form. Instead of hollow balls, however, any different shapes of supporting bodies can be used as well. As a result, hollow balls will not develop but hollow shaped bodies will develop in the form of the respectively used supporting bodies. In the present description, hollow balls and hollow shaped bodies are to understand as equivalents in principle as far as the explanations on the prior art do not really relate to hollow balls.

[0004] According to the prior art it is known to deposit enveloping layers on a supporting element, and to sinter this enveloping layer. On that occasion, the supporting element is pyrolyzed just before reaching the densification temperature, and the respective substance is expelled through the enveloping layer. After sintering, a hollow shaped body is developing which is very lightweight and has a relatively high strength.

[0005] In U.S. Pat. No. 3,674,461 hollow spherical particles made of aluminium, magnesium, boron and beryllium are mentioned which are free of holes and seams and the diameters thereof are smaller than about 4.5 mm wherein the wall thickness is less than about 0.2 mm. For building-up the particles, the respective material is built-up in the form of a powder coating upon a core. On that occasion, it is mentioned that the cores are filled into a rotating vessel in which a metered quantity of the powdery coating material is located. The core is comprised of naphthalene, anthracene, camphor or polyaldehyde, for example. Subsequently, the core is sublimed in vacuum over a longer period and removed gaseously through the coating. Finally, the remaining hollow balls, e.g., made of aluminium are oxidized at temperatures ranging from 700 to 800° C. As a result, shells of ceramics of the respectively used starting materials are developed.

[0006] In U.S. Pat. No. 3,792,136 a method for the production of high-grade porous hollow metal oxide balls made of a metal oxide from the group of silicon, aluminium, calcium, magnesium and circonium oxides is described. With this, e.g., epoxy resin balls having a diameter of 2 to 4 mm are soaked with an oxidizable salt solution of the mentioned metal with ammonium hydroxide. The epoxy resin balls soaked with metal oxide are dried and carbonized. Thereafter, the balls treated in this way are treated in an oxidizing atmosphere such that the resin is expelled by decomposition as well as the carbon is expelled by oxidation, and the provided metal oxide is formed. The metal oxide balls have a porous structure in all.

[0007] In DE 36 40 586 A1 a method of the production of hollow balls or hollow ball compounds having walls with increased strength is mentioned. On that occasion, further layers are deposited on metallized spherical lightweight body particles having a core of foamed polymer and a metal wall thickness of 5 to 20 μm. The metallized spherical lightweight body particles are coated with a dispersion of fine part metal, the oxide thereof or fine part ceramic or fire-proof material. The layer thickness is to come to 15 to 500 μm. The coated lightweight body particles are dried, the polymer core is pyrolyzed at 400° C. and subsequently sintered at 900 to 1 400° C. As a result, hollow balls having a dense or porous wall are developed according to the particle size, type and densification temperature of the non-metallic material. When sintering is carried out in moulds then correspondingly shaped hollow ball compounds made of sintered metallic or ceramic hollow balls are formed immediately. The cellular walls having a wall thickness ranging from 15 to 500 μm are supposed to have an increased strength.

[0008] In EP 0 300 543 A1 a method for producing metallic or ceramic hollow balls is described in which a solid matter layer is deposited upon an essentially spherical particle made of foamed polymer, and the coated polymer core is pyrolyzed. Then, when moved the spherical particles are treated with an aqueous suspension which contains a dissolved and suspended binding agent and metallic and/or ceramic powder particles. The coated and dried particles are pyrolyzed at 400 to 500° C. when moved, and sintered at temperatures of 1 000 to 1 500° C. when moved.

[0009] According to the prior art, hollow balls can be produced which diameters thereof are practically within 0.5 and 5 mm. With such hollow balls completely sintered structures or practically employable shaped portions can be produced the mass of which can be decreased up to 3%, preferably up to 1% with respect to the mass of the respectively used massive material.

[0010] The size of the used powder particles is particularly important for the strength of the hollow balls. The ratio of strength and lightweight of the hollow balls and the structures produced therefrom is essentially determined by the ratio of ball diameter and thickness of ball wall. The optimum wall thickness of the hollow balls should be appr. 0.5 to 3% of the outer ball diameter. In most cases, the wall thickness is about 1%. Hollow balls with a diameter of 5 mm have a wall thickness of about 50 μm in accordance with it, with a ball diameter of 1 mm there are only 10 to 20 μm yet, and with balls having a diameter of 0.5 mm there are only 5 to 15 μm at maximum of wall thickness yet.

[0011] The minimum size of the styrene balls used in practice as supporting material which determine the inner diameter of the hollow balls is limited to about 0.8 mm. Smaller styrene balls are not fabricable. With supporting materials other than spherical ones the relations are the same. By coating the styrene balls the diameters thereof are still increasing as can be expected. If metallic hollow balls being smaller than 0.8 mm are to be produced, plastic balls being not foamed ought to be used. Thereby, though the quantity of the plastic to be pyrolyzed is increasing so strong such that expelling the material of the ball core becomes impossible in an economic and environmental helpful manner.

[0012] To ensure an appropriate high strength of the ball wall powder particles each are to be used which have significantly less outer dimensions than the thickness of the hollow ball wall. On the other hand, the powder particles are allowed to sinter to each other only on a few lateral contact points within the wall structure. Regularly, the medium size of the powder particles should not be greater than 10% of the thickness of the spherical wall. That means, with the production of hollow balls having an outer diameter of 1 mm it is required to have a metal powder with a medium particle size of 1 μm. Such metal powders are not commercially available at least if made of relatively low cost metals such as iron and copper. Admittedly, the production of metal powders with particle sizes in the range of nanometers is possible, however, these powders have a great avidity, and hence can only be processed with great expense. Moreover, these powders are such expensive that materials produced therefrom become of no interest to large-scale applications for price reasons.

[0013] The most fine-grained commercially available iron powder is carbonyl iron powder, which has a medium particle size in the range of 5 μm and is only suitable for the production of wall thicknesses of more than 20 μm. Metallic hollow balls ranging from 2 to 4 mm are such expensive due to the high price of carbonyl iron powder such that they cannot pass the competition with other comparable lightweight structures. Smaller hollow balls are mentioned in the literature indeed, however, in practice they are not fabricable according to the prior art.

[0014] With the practical application of the hollow balls and hollow shaped bodies, respectively, particularly in the solid structures or structural members the homogeneity of the hollow ball structure is substantially determined by the size of the hollow balls. Therefore, the resistance to pressure which can be practically realized, and the homogeneity of the properties of sintered hollow ball compounds will be limited by the size of the smallest available hollow balls. Admittedly, the resistance to pressure of a hollow ball compound can be increased by compaction pressing, however, the density of the hollow ball compound then also increases in a mostly undesirable manner, and the effect of lightweight structure being desired in principle is lost again.

[0015] The invention is based on the object to provide metallic, miniaturized hollow shaped bodies for the advantageous application of such shaped bodies in particular in the structural members or semifinished products having a high resistance to pressure. Furthermore, the object is to provide a method by means of which metallic shaped bodies can be produced.

[0016] In accordance with the invention, the object for the metallic miniaturized hollow shaped bodies is solved by the features mentioned in the characterizing part of claim 1. In accordance with the invention, the object for the method is solved by the features mentioned in the characterizing part of claim 7. Advantageous improvements are characterized in the respective subclaims and will be shown in more detail in the following together with the description of the preferred aspect of the invention.

[0017] The gist of the invention is particularly in that the miniaturized metallic hollow shaped bodies, which for the sake of simplicity are subsequently called only hollow balls as well and are comprised of at least one heavy metal which can be reduced from a respective metallic compound in an atmosphere containing hydrogen or carbon at a temperature of less than 1 500° C., preferably less than 1 200° C. Fe, Ni, Co, Sn, Me, Cr, Cu, Ag, Pd and W are used as such a heavy metal. The metallic shaped bodies have an outer diameter ranging from 0.05 to 0.5 mm, and the diameter-to-wall thickness ratio ranging from 0.5 to 3%.

[0018] The individual metallic shaped bodies can also be constituted of alloys of the metals mentioned, and/or the wall of the shaped bodies can be constituted of similar or dissimilar materials.

[0019] When used, the metallic miniaturized hollow shaped bodies are allowed to be sintered in shaped body compounds to structural members or semifinished structural members.

[0020] The shaped bodies according to the invention result in a great number of sintering points in the shaped body compound. The shaped body compounds are very homogeneously and have a very high resistance to pressure. The shaped body compounds can be worked up well in metal-cutting and without cutting manners, and the homogeneous structure allows the use of jointing methods such as screwing and nailing as well.

[0021] The density of the shaped body compounds is in principle maintained. According to the selected diameter-to-wall thickness ratio, the density of the shaped body compounds can be further decreased with respect to the prior art. The surfaces of the shaped body compounds have a low roughness.

[0022] The shaped bodies according to the invention are not fabricable with means of the prior art. Therefore, a hew method in accordance with the invention for the production of the novel metallic miniaturized hollow shaped bodies is mentioned.

[0023] For the production of metallic miniaturized hollow shaped bodies, in accordance with the invention a method is used according to the preamble of claim 7, in which essentially reducible metallic compounds preferably metal oxides, metal hydroxides, metal carbonates or organometallic compounds (erg. acetates, formiates, oxalates, acetylacetonates) are selected as starting materials for building-up the enveloping, layer on the supporting element.

[0024] The coated supporting elements are sintered with an enveloping layer containing at least such one metallic compound (so-called green compacts) during the thermal treatment in a reductive atmosphere such that the starting materials are essentially reduced to the sintered metal which is based on the respectively used metallic compound.

[0025] In one enveloping layer at least two compounds of different heavy metals can also be contained which form an alloy during sintering under reductive conditions.

[0026] The enveloping layer can be formed with a plurality of layers wherein the same metallic compound(s) or also different metallic compounds can be included in the individual layers.

[0027] The metallic compounds can be partially used at least in a colloidal form. It is also possible to use a part of the metallic compounds dissolved in a liquid, preferably water.

[0028] The medium particle size of reducible metallic compounds as starting materials should be far below 5 μm but also be included in a colloidal form in a liquid, if possible. The starting materials such as technical chemicals or as pigments for the dyestuff industry having very low particle sizes are often available substantially more inexpensive than comparable metal powders. Iron oxides for the use as a pigment, for example, are commercially available in the ranges of 500 nm to less than 100 nm. With respect to the metals a plurality of metallic compounds are very brittle such that they can be milled to a medium particle size in the range of 1 μm in ball mills with low cost. This is impossible with metals due to the ductility thereof.

[0029] In the practice, metal powders ranging below 0.01 mm are very expensive and are not available.

[0030] According to the object of the invention, as supporting elements are regularly used such ones having a diameter of less than 1 mm.

[0031] During the thermal treatment of the green compacts at first the material of the supporting elements is pyrolyzed and expelled from the balls in a well-known manner. During sintering in a reductive atmosphere the metallic compounds are transformed into the respective metal which is based on the respectively used metallic compound. During sintering, it is of advantage to use a reductively acting protective atmosphere such as hydrogen, cracked ammonium gas, exothermic atmosphere or endothermic atmosphere. Thus, oxides developing during the thermal decomposition as well, can be reduced into metal.

[0032] With sintering the shaped bodies and a shaped body compound, respectively, an approach of the metal particles and thus a contraction of the shaped bodies and the shaped body compound, respectively, commences by expelling the respective substances during the reduction.

[0033] On that occasion, the outer diameter of the individual hollow ball as well as the thickness of the ball wall are reducing. The lower the particle size of a starting material to be sintered the greater the contraction. Even with this respect, a low particle size of the metallic compound has a favourable effect on the miniaturization.

[0034] The second effect being often stronger, which has a favourable effect on an increased contraction and thus miniaturization is the fact that a metallic compound always comprises a lower relative density, and thus occupies a greater volume than the metal itself.

[0035] This shall be clarified in more detail with two examples. Fe₂O₃ has a density of 5.2 g/cm³. It is comprised of 69.9 percent in weight of iron. From 100 cm³ of Fe₂O₃ only 46 cm³ of metallic iron are developing by means of reduction. Nickel hydroxide has a density of 4.15 g/cm³. It is comprised of 63 percent in weight of nickel. From 100 cm³ nickel hydroxide appr. 29 cm³ of metallic nickel are developing by means of reduction.

[0036] The respective material specific dimension of contraction can be exactly precalculated.

[0037] In addition to the already described increased resistance to pressure of the finished shaped body compounds there are another advantages. The surface roughness of structures or structural members is substantially reduced. Thereby, surfaces are developing which can be usually denoted as smooth surfaces. Due to the smaller outer diameters of the hollow balls the structure of a hollow ball compound becomes more homogeneously in all, and the mechanical properties will be improved. The shaped body compounds can be easily worked up in the metal-cutting and without cutting manners. For example, nails or screws can be inserted as well.

[0038] In the following, the invention will be explained in more detail with two embodiments.

Embodiment I

[0039] In the embodiment I hollow iron balls having a medium diameter of appr. 0.5 mm and a wall thickness of appr. 10 μm are to be manufactured.

[0040] According to the method, an enveloping layer made of a suspension comprising a liquid, a binding agent and a red colour pigment of Fe₂O₃ having a medium particle size of 0.32 μm is built-up on 1 liter of styrene foam balls having a medium diameter of 0.8 mm The thickness of the enveloping layer is appr. 20 μm. The styrene foam balls coated in this manner are denoted as green compacts.

[0041] The diameter of the green compacts is appr. 0.84 mm, and the volume is increased appr. 10% from 1 liter to appr. 1.1 liters. After thermal treatment in a hydrogen atmosphere at temperatures of appr. 1 150° C. the organic components of the green compact are burnt out. The iron oxide is reduced, and a sintered hollow ball is forming.

[0042] After sintering, from the appr. 1.1 liters of green compacts results appr. 0.6 liter of metallic hollow iron balls having a medium diameter of appr. 0.3 mm, and a wall thickness of appr. 10 μm.

Embodiment II

[0043] According to the method, an enveloping layer made of a suspension comprising a liquid in which a binding agent and nickel acetate as well are dissolved, and a powder of nickel hydroxide having a medium particle size of appr. 500 μm is built-up on 1 liter of styrene foam balls having a medium thickness of 0.5 mm. The thickness of the enveloping layer is appr. 15 μm. The diameter of the green compacts is appr. 0.53 mm, and the volume is increased from 1 liter to appr. 1.2 liters. After thermal treatment at 400° C. in inert gas the organic and other volatile components of the green compact are burnt out, and with a subsequent thermal treatment in a hydrogen atmosphere at temperatures of 1 120° C. the resulting nickel oxide is reduced, and a sintered hollow ball of nickel is forming.

[0044] After sintering, from appr. 1.2 liters of green compacts are developing appr. 0.5 liter of metallic hollow balls of nickel having a medium diameter of 0.1 mm and a wall thickness of appr. 2 μm.

[0045] Of course, the invention is not limited to the described embodiment.

[0046] Thus, it is readily possible to combine the method according to the invention with further well-known method steps or to use the method for the production of hollow balls the outer diameter of which is greater than 0.5 mm.

[0047] For example, the supporting bodies can be radially formed. These are advantageously manufactured by means of an extruder wherein the original extruder castings are subsequently disintegrated. 

1. Metallic miniaturized hollow shaped bodies, characterized in that said metallic shaped bodies are comprised of at least one heavy metal which can be reduced from a corresponding metallic compound at a temperature of less than 1 500° C., and that said metallic shaped bodies have an outer diameter ranging from 0.05 to 0.5 mm and a diameter-to-wall thickness ratio ranging from 0.5 to 3%.
 2. Metallic shaped bodies according to claim 1, characterized in that said metallic shaped bodies are comprised of at least one heavy metal, which can be reduced from a corresponding metallic compound at a temperature of less than 1 200° C.
 3. Metallic shaped bodies according to claim 1 or claim 2, characterized in that said metallic shaped bodies are comprised of Fe, Ni, Co, Sn, Mo, Cr, Cu, Ag, Pd or W.
 4. Metallic shaped body according to any one of claims 1 to 3, characterized in that said metallic shaped bodies are comprised of an alloy.
 5. Metallic shaped bodies according to any one of claims 1 to 4, characterized in that the wall of said metallic shaped bodies are formed in a multilayered manner.
 6. Metallic shaped bodies according to any one of claims 1 to 5, characterized in that said metallic shaped bodies are sintered to structural members or semifinished products.
 7. A method for the production of metallic miniaturized hollow shaped bodies in which the starting materials are deposited as an enveloping layer upon supporting elements of any shape, said green compacts thus produced are subsequently heat-treated such that said supporting elements are pyrolyzed and said enveloping layers are essentially thermally decomposed, and the decomposition products are sintered, characterized in that supporting elements will be selected which have outer dimensions which are greater than said shaped bodies to be produced, that metallic compounds, preferably metal oxides, metal hydroxides, metal carbonates or organometallic compounds are selected as starting materials and can be reduced at a temperature of less than 1 500° C., and that said green compacts are heat-treated in a reductive atmosphere containing hydrogen and/or carbon such that said metallic compound is essentially reduced to metal, and said metal is sintered.
 8. A method according to claim 7, characterized in that metallic compounds are selected which can be reduced at a temperature of less than 1 200° C.
 9. A method according to claim 8 or claim 9, characterized in that as a metallic compound is selected such one which is based on the metals of Fe, Ni, Co, Sn, Mo, Cr, Cu, Ag, Pd or W.
 10. A method according to any one of claims 7 to 9, characterized in that at least two metallic compounds forming one metal alloy are selected.
 11. A method according to any one of claims 7 to 10, characterized in that powder metallic compound(s) is (are) used.
 12. A method according to any one of claims 7 to 11, characterized in that said starting materials are used at least partially in a colloidal form or a dissolved form.
 13. A method according to any one of claims 7 to 12, characterized in that said starting materials are deposited in a multilayered manner on said supporting bodies.
 14. A method according to any one of claims 7 to 13, characterized in that said thermal treatment of said green compacts is carried out in a mould for the formation of structural members or semifinished structural members.
 15. A method according to any one of claims 7 to 14, characterized in that as supporting bodies are selected such ones which have been produced by means of extruders wherein the original extruder castings are subsequently disintegrated. 